Does a piece of skeletal muscle, transplanted to the brain's surface, provide an entry into the brain for blood-borne macrophages? Pieces of neck muscle, 2-3 mm x 1 mm, are grafted to the dorsal surface of the medulla in mature rats. Two to 12 weeks later, the rats macrophages are withdrawn from the peritoneal activity, pelleted and labeled with a long-lasting fluorochrome "Cell linker," which is presumably a carbocyanin dye. The macrophages are then infused as a single bolus into the rat's axillary artery. This vessel, which has not been used heretofore as a means of perfusing the vertebral artery, leads the cells into the brain stem circulation. A few labeled macrophages accumulate in the graft and surrounding choroid plexus; very few enter the medulla under the muscle graft. It is to be tested whether macrophages will enter the brain primarily from the intrinsic capillaries of the CNS or from the graft, when endotoxin is perfused through the cerebral ventricles prior to introducing the macrophages.